Nucleophilic metabolites, conjugation

For N-methyl arylamines, electrophilic N-sulfonyloxy esters appear to be strong candidates for the ultimate carcinogenic metabolites. However, additional studies are required as these conclusions are circumstantially based on their comparative reactivity with nucleophiles and on the failure of other metabolic conjugation systems to esterify N-hydroxy-N-methyl arylamines (9,187). 

Most reactive metabolites produced by CYP metabolic activation are electrophilic in nature, which means that they can react easily with the nucleophiles present in the protein side chains. Several functional groups are recurrent structural features in M Bis. These groups have been reviewed by Fontana et al. [26] and can be summarized as follows terminal (co or co — 1) acetylenes, olefins, furans and thiophenes, epoxides, dichloro- and trichloroethylenes, secondary amines, benzodioxoles (methylenediox-yphenyl, MDP), conjugated structures, hydrazines, isothiocyanates, thioamides, dithiocarbamates and, in general, Michael acceptors (Scheme 11.1). 

Several other minor metabolites of phenol have been identified in vitro. The formation of 1,4-dihydroxybenzene and 1,2-dihydroxybenzene in a 20 1 molar ratio was observed in isolated rat liver microsomes incubated with phenol (Sawahata and Neal 1983). Further catalysis to / -benzoquinone, 4,4 -biphenol, and biphenoquinone has been demonstrated in microsomes and in in vitro peroxidase preparations. The benzoquinone products react nonenzymatically with nucleophiles, including cysteine and reduced glutathione, to yield. V-conjugates of 1,4-dihydroxybenzene and 4,4-biphenol (Eastmond et al. 1986 Lunte and Kissinger 1983 Subrahmanyam and O Brien 1985). 

Preliminary mechanistic studies show no polymerization of the unsaturated aldehydes under Cinchona alkaloid catalysis, thereby indicating that the chiral tertiary amine catalyst does not act as a nucleophilic promoter, similar to Baylis-Hilhnan type reactions (Scheme 1). Rather, the quinuclidine nitrogen acts in a Brpnsted basic deprotonation-activation of various cychc and acyclic 1,3-dicarbonyl donors. The conjugate addition of the 1,3-dicarbonyl donors to a,(3-unsaturated aldehydes generated substrates with aU-carbon quaternary centers in excellent yields and stereoselectivities (Scheme 2) Utility of these aU-carbon quaternary adducts was demonstrated in the seven-step synthesis of (H-)-tanikolide 14, an antifungal metabolite. 

Glutathione and sulfatation conjugations are important pathways for generally detoxifying endogenous substrates and xenobiotics (Commanndeur et al. 1995). However, some produced metabolites (ie mercapturic acids, O-sulfo conjugates) are toxic by different mechanisms, often by reaction with DNA and other cellular nucleophiles. 

Electrophiles not only complex with macromolecular nucleophilic sites, but also complex with those of small molecules. The most important of these is the -SH group of GSH. Conjugation of electrophiles with GSH is an important detoxication reaction catalyzed by GSH transferases (see Chapter 12). Although GSH concentration in hepatocytes is high relative to other tissues (4-8mM), depletion of GSH can occur upon acute exposure to protoxicants whose electrophilic metabolites... 

In some cases. Phase I metabolites may not be detected, owing to their instability or high chemical reactivity. The latter type are often electrophilic substances, called reactive intermediates, which frequently react non-enzymically as well as enzymically with conjugating nucleophiles to produce a Phase II metabolite. A common example of this type is the oxidative biotransformation of an aromatic ring and conjugation of the resulting arene oxide (epoxide) with the tripeptide glutathione. Detection of metabolites derived from this pathway often points to the formation of precursor reactive electrophilic Phase I metabolites, whose existence is nonetheless only inferred. 

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